The present invention concerns an easily-slidable polyarylene thioether film not containing a substantial amount of powdery lubricant, as well as a production process therefor.
Specifically, the present invention relates to an easily-slidable film obtained by treating with an organic solvent at least one side of a substantially amorphous film comprising, as a basic resin, polyarylene thioether having a repeating unit of ##STR2## as a main constituent and not containing a substantial amount of powdery lubricant, to make the crystallization degree of its surface layer higher than that of its inside and then stretching and heat treating the solvent-treated film to make an arithmetic average surface roughness, Ra, of 2 to 200 nm.
The present invention also relates to the production process thereof.
Hitherto, in the field of electronic and electric products, particularly, film-like products such as magnetic recording materials, capacitor films and insulation films, various kinds of synthetic resin films, among all, easily-slidable films have been used in a great amount as base films. Particularly, in the field of heat resistant and easily-slidable film, polyester films or polyimide films have been employed as the base films.
These films have been useful as described above. However, recording materials vapor-deposited with magnetic material or chip capacitors have been marketed remarkably in recent years and it has been pointed out that polyester films are insufficient in its heat resistivity to such applications. Accordingly, easily-slidable films having higher heat resistivity have been desired. While on the other hand, there has been a problem in the polyimide films which are extremely excellent in its heat resistance that they are extremely costly, difficiult for melt formation of film and have large humidity expansion coefficient. Therefore, they are not suitable for general purposes.
In view of the above, an easily-slidable base film comprising polyarylene thioether (hereinafter simply referred to as PATE) has been disclosed as one of potential candidates for the easily-slidable base films which can easily be formed into films by melt processing and have well balanced properties such as heat resistance, mechanical property, dimensional stability and economical advantage in Japanese Patent Application Laid-Open (KOKAI) No. 55-34968 (1980). However, as far as the present inventors recognize, there has been a problem that preparation of an easily-slidable base film with small arithmetic average surface roughness and without abnormally coarse projections from the surface (for example, films for vapor deposition) is difficult by the disclosed method of adding powdery additives such as calcium carbonate, since defects or abnormally coarse projections from the surface of the film are liable to be formed. Further, there is a technical limit for preparing powdery additives of small size and it is difficult to disperse such additives of small size uniformly in a film with no agglomeration. Furthermore, there has also been a problem in the method that it is almost impossible to make only one surface slidable.
As a method of producing an easily-slidable film without using powdery additives, it has also been known to mechanically roughen a film surface by pinching with a rough-surface-roll. However, there has been problems that it is difficult to prepare an easily-slidable film with a small arithmetic average surface roughness by such a mechanical method. Therefore, a film prepared by such mechanical method is not suitable for the use, where an easily-slidable film of particularly small arithmetic average surface roughness is required, for instance, applications for magnetic tapes or magnetic floppy discs prepared by vapor deposition.
As a method of producing a PATE base film capable of satisfying both conditions that only one side can be made easily-slidable and that its arithmetic average surface roughness is small and substantially no abnormally coarse projections from the surface are present, there may be a method of coating a rough surface layer on a film having a smooth surface.
However, such a method involves problems that the operation of winding or rewinding the smooth surface film to and from a roll or a machine for the surface coating is difficult because such films easily stick together and that the binders have poor heat resistivities.
The present inventors have made an extensive study for overcoming the foregoing problems and, as a result, have found that an easily-slidable PATE film capable of dissolving the foregoing problems altogether can be obtained by utilizing fine projections of PATE per se formed on the film surface instead of using a powdery additive and have accomplished the present invention on the basis of such a finding.